Many types of mass spectrometers are known, and are widely used for trace analysis to determine the structure of ions. These spectrometers typically separate ions based on the mass-to-charge ratio (“m/z”) of the ions. One such mass spectrometer system involves mass-selective axial ejection—see, for example, U.S. Pat. No. 6,177,668 (Hager), issued Jan. 23, 2001. This patent describes a linear ion trap including an elongated rod set in which ions of a selected mass-to-charge ratio are trapped. These trapped ions may be ejected axially in a mass selective way as described by Londry and Hager in “Mass Selective Axial Ejection from a Linear Quadrupole Ion Trap,” J Am Soc Mass Spectrom 2003, 14, 1130-1147. In mass selective axial ejection, as well as in other types of mass spectrometry systems, it will sometimes be advantageous to control the axial location of different ions.